The flexible packaging of foodstuffs, medical products, industrial goods, and the like is a growing and important area of commerce. The creation of flexible packaging materials is usually a multi-step process. Converters, or the producers of flexible packaging films, are companies that typically initially print flexible films, laminate, slit and supply the flexible web stock to an end-user. Such web stocks are then chosen for printability, barrier properties, clarity, scuff resistance, heat-sealability, and several other technical and cost considerations for use in the final product. The web stocks, after creation and selection, are then further processed on a product packaging line to create a pouch, bag, tray, lid, or similar structure at the point of use. That creation of the end-use package subsequently allows for an increased freshness or shelf-life extension for many commodities. Indeed, it is a goal within the industry to increase shelf-stability for a range of food and medical materials, while also presenting an appealing packaged product to the consumer.
In this regard, within the flexible packaging area, there is often a need to create formable structures from a flat web. The creation of a well, or cavity, is a well-known packaging methodology to provide for the easy packaging of meats, medical devices, drugs, and other materials. For instance, the creation of a cavity, or well, can be accomplished through a combined heat and reduced pressure process that molds the web to cavity, whereby the cavity is designed to provide the overall volume needed for packaging the targeted commodity. See, for example, U.S. Pat. No. 3,496,143, which is incorporated herein by reference.
Heating of a cavity mold can be performed by thermal, induction, or radiant sources. Thermal heating, however, is the most preferred methodology due to simplicity and cost considerations. Furthermore, the creation of the cavity from a thermoforming web-stock can be assisted with vacuum application. The negative pressure on the bottom side of a flat web over a mold orifice provides a driving force to distort the web stock into the final desired shape. Creation of the cavity based on size and shape is performed more readily with vacuum thermoforming.
Other common methods of cavity formation include the use of a pressure piston to form the desired shape. Flat web-stock may be inserted into the mold press and under pressure such that a permanent well is formed in flexible films and laminates. Indeed, such cold forming of aluminum foil containing laminates is a common methodology for the production of blister materials in the pharmaceutical markets. For instance, U.S. Pat. No. 4,537,312, which describes the construction of foil blister packaging that is particularly suited for tamper evident pharmaceutical packaging.
Within the blister packaging technical area, high barrier is often necessary and aluminum foils are not susceptible to thermoforming. Thus, the use of cold forming techniques is prevalent. These methods are typically referred to as cold formed foils or CFF. A particularly important forming structure in this area consists of nylon films laminated to aluminum foils and then again to polyvinylchloride films (PVC). Such structures, when formed into blisters, have advantageous product protection, product integrity, and compliance attributes as described in Pharmaceutical Technology, November 2000, pages 66-77, which is also incorporated herein by reference.
Commonly, within the flexible packaging technical field, nylon films have been used in combination with aluminum foil to create many cavity structures. In this regard, it is appreciated that nylon films are known to have high elongation properties and are thus well suited for thermoforming and cold forming processes. Within the molding process itself, it is often necessary for the flat web materials to be able to stretch and distort uniformly under heat and pressure to the desired mold volume. Rheological properties of the material dictate the amount of deformation under an applied stress, strain recovery after elimination of the applied load, and permanence of the strain. For many materials including PET, copolyesters, and blends of PET and miscible components, tan delta values from torsional stress/strain experiments are typically higher at low temperatures and are maximized at the glass transition temperatures of the materials. The importance of these principles to thermoforming can be found, for example, in “Importance of elongational properties of polymer melts for film blowing and thermoforming”; Polymer Engineering & Science Volume 46, Issue 9, pages 1190-1195, September 2006, which is further incorporated herein by reference.
A typically desired input web may be any combination of printed material, barrier webs, adhesives, sealable materials, and the like. In this regard, it is sometimes considered important that all components of the flexible web have the capability to distort uniformly into the mold structure, as differences in moldability between the discontinuous web-stocks of the flexible web can cause molding issues like splitting, uneven distortion, crystallization, or other defects capable of diminishing the suitability of the molded web for the end-use application.
Within the flexible film and flexible packaging technology areas, the use of thermoplastic materials like polyethylene, polypropylene, nylon, polystyrene, polyethylene terephthalate (PET), polylactic acid, and other thermoplastic commodities to produce films is appreciated. Each base polymer, or resin, has intrinsic technical attributes like barrier properties, optical clarity, hardness, surface energy, softness, etc. that makes their selection for the end-use need appropriate. Extruding these thermoplastic polymers into web structures and orienting them into thin films is an important industrial process to induce further enhanced properties into polymeric materials. Stretching and orientation are appreciated in the art to improve tensile and elongation properties, tear properties, scuff resistance, etc. Therefore, it is also appreciated in the art to select a base resin and filming process capable of creating a suitable film substrate for the technical needs in a flexible packaging web.
Among films currently available, PET is a material with excellent barrier, clarity, printability, and hardness properties. Film forming and orientation of that material can be used to create thin profile webs with excellent properties for use in flexible packaging. Typically, such PET films have high thermal stability and low subsequent moldability properties. In fact, although traditional biaxially-oriented PET films can be produced with subsequent down-stream moldability, the moldability is generally not high. Alternatively, however, high thermoforming with PET films can be achieved if the films are not oriented, or are monoaxially oriented as described, for example, in U.S. Pat. No. 4,073,857, which is also incorporated herein by reference.
Despite the advantageous properties of PET films, when thermoforming oriented PET web material under heat and pressure, current PET film structures and converted webs will often split or break easily. Due to the limitations of current PET films, the utility of these materials for the production of thermoformed trays, wells or cavities has thus been limited. Converters, therefore, often need to select other film materials when the packaging structure involves a thermoformable structure.
To resolve such low formability issues of current PET film materials, there has been research into blends of PET with other materials to improve the molding process. Such techniques, however, although improving the thermo-molding properties, have only created other issues such as increased cost, regulatory clearance issues, optical clarity problems, and recycling issues. Accordingly, there remains a need in the art for PET film structures capable of easy forming and use in a range of end-use packaging requirements, including thermo-formed and cold-formed wells, trays, or cavities, and for a range of applications, including foodstuffs, medical products, and industrial goods.